This invention relates to a method for recovering polymer from solution. More particularly, it relates to a continuous process for isolating polymer resins from solution by admixing the solution with an aqueous slurry of solid polymer particles and devolatilizing the organic solvent.
There are several methods which have been suggested for separating thermoplastic resins from organic solutions. The majority of these processes are energy intensive, requiring large quantities of steam to accomplish solvent removal. Those separation processes which are energy conservative either provide solid product of a low bulk density, provide particulate product with undesirable impurities or require complex operating equipment.
One example of an energy intensive process is that disclosed in U.S. Pat. No. 3,590,026 where solvent is removed from a rubber solution in a two-stage stripping system. In addition to the great amount of energy consumed as steam, the process requires a dispersant, which may be retained as a contaminant in the final polymeric material.
Other methods which provide undesirable particulate products are disclosed in U.S. Pat. Nos. 3,954,713 and 4,205,162. The process of U.S. Pat. No. 3,954,713 separates polycarbonate from an organic solution without the use of excessive quantities of steam by forming an emulsion of the polycarbonate solution in water which contains a "laminator", a high molecular weight polymer which increases the viscosity of water and reduces its resistance index. The threat of residual laminator remaining in the final product makes this process undesirable. The process of U.S. Pat. No. 4,205,162 separates a polymer product from solution by the addition of hot water with mild agitation. Due to the high rate of solvent removal, the particles obtained are finely divided, fluffy flakes which have a low bulk density. Such a morphology is undesirable for further processing of the polymer.
The process described by Kohyama et al in U.S. Pat. No. 4,452,976 obtains polymer products of high density by feeding a polymer solution into water in conjunction with a recycled product slurry which has been treated by a wet pulverization process. The polymer solution is then devolatilized within the water/product slurry. This process is very energy intensive in that it requires a high recycle rate to prevent clogging of the apparatus. In addition, complex equipment is required (wet pulverizer) to perform the process.
The processes described in U.S. Pat. Nos. 4,408,040 and 4,423,207 provide methods for separating polymers from solution wherein said solution is atomized with steam within an agitated vessel of hot water. The solvent is flashed off within the hot water leaving a slurry of solid polymer. These processes provide an energy efficient means for separating the polymer from solution; however, there is room for improvement. For example, these processes require unique equipment, such as a spray nozzle designed to disperse the polymer. In addition, a polymer particulate of a higher bulk density, a narrower size distribution and a lower concentration of retained volatiles is desired. A method which obtains these improvements in less complicated equipment is desired.